The invention is directed to a transfer station for an electrographic printer or copier means having a pressure element for the recording medium arranged in the transfer region.
Two-sided, electrographic printing of an elongate recording medium is disclosed, for example, in U.S. Pat. No. 5,179,417 which discloses that two printers be operated in tandem mode. The printers are thereby connected following one another, whereby the first printer of the arrangement prints the front side and the second printer prints the back side of the recording medium. Among other things, each printer contains a transfer station for transferring loose toner images from a photoconductive drum onto the recording medium and also contains a thermal fixing station in which the loose toner image is fixed on the recording medium by the influence of pressure and heat.
In the thermal fixing process, the recording medium, which is usually composed of paper, is conducted through the printer between a heated fixing drum and a pressure roller, where the toner image is heated and is fused into the recording medium. The paper is thereby heated up into the range of 200.degree. C. This leads to a shrinkage of the paper due to gas evolution and may potentially result in local warping and deformation. Since, in tandem printing mode, the recording medium again passes through the transfer station in the second printer for printing the back side, such warpings have a negative influence on the transfer quality. The paper does not have its entire width lying against the photoconductive drum during transfer printing, as a result a local deficient transfer of the tonal particles in the transfer region can occur.
Such electrographic printer equipment are often operated with pre-folded continuous stock having a great variety of paper weights and formats. It is thereby desirable to potentially print the individual sheets up to the region of the fold. The elevations and warping of the fold likewise have a negative effect on the transfer process.
In order to reduce, among other things, the negative influence of such warping of the recording medium on the transfer printing quality, said U.S. Pat. No. 5,179,417 discloses that a one-piece paper pressure element of plate-shaped polyamide plastic be arranged in the transfer region. A paper pressure element fashioned in this way has the disadvantage that it is difficult to compensate small local elevations and warps. Moreover, it is contaminated by paper abrasion and the like. Since the element is rigidly joined to the transfer station, the entire transfer station must be taken apart in case of malfunction given damage or contamination.
It has been observed surprisingly that such a pressure element allows a "caries effect" to occur in the transfer of the toner images. A letter or a line is thereby not completely transferred and white, uninked regions remain within the characters or surfaces. This effect is shown in FIG. 7. Reference 31 thereby indicates the printed character and reference 32 indicates the white, uninked blank locations.
The inventor has discovered that the electrostatic charging of the paper in the region of the pressure element is the reason for this effect.